Xanthan gum is an anionic polysaccharide produced by the submerged fermentation of a xanthan gum producing bacterium of the genus Xanthomonas. It is used in many industrial fields, including, foods, cosmetics, medicines and the like. In the conventional fermentation processes used for the preparation of xanthan gum, bacteria other than the xanthan-producing bacteria may grow which may survive the process and end up as live bacterial contaminants in the final xanthan product.
In certain of these end uses, and in particular, in food, cosmetic, and medical products, it is preferable that the number of contaminative live bacteria in the xanthan gum decrease as much as possible. Preferably, the number of the bacteria is about 100 or less per gram of the xanthan gum.
In conventional xanthan processing, after the fermentation is completed, an organic solvent in which xanthan is insoluble, typically, isopropanol, is used to precipitate the xanthan from the fermentation broth. Usually the amount of isopropanol used to precipitate the xanthan gum is 1 to 3 times by volume of the volume of the fermentation broth containing the xanthan gum. The precipitated xanthan gum is collected, dehydrated and then directly dried.
In certain procedures for the recovery of xanthan from fermentation broths, the broth may be subjected to a heating step before the precipitation procedure. See, for example, Japanese Provisional Patent Publication 2-60597 which relates to a method for enhancing the efficiency of filtration by sterilizing bacteria by heating after culturing and agglomerating the bacteria.
The xanthan gum producing bacteria are killed by such a heat treatment or by contact with the isopropanol used to precipitate the gum. However, contaminative bacteria can get into the xanthan gum, for example, in a process line, subsequent to the addition of the isopropanol. Contaminative bacteria entering at this point are not killed by the isopropanol present or the heat treatment and end up in the xanthan product as live bacterial contaminant.
Sterilization methods for the reduction of the number of contaminative bacteria in the xanthan gum have been suggested. Such sterilization methods include those described in U.S. Pat. No. 3,206,275 wherein a method for the sterilization of materials, such as, tobacco, spices, rice and the like, by subjecting them to repeated gas treatments, is disclosed. U.S. Pat. No. 3,919,189 discloses a method for sterilization of particulate xanthan gum by intermingling it with gaseous propylene oxide.
However, these methods have some drawbacks in that they do not achieve sufficient sterilization or destruction of the bacterial contaminants. Also, the aqueous xanthan gum solution exhibits decreased transparency and the xanthan gum exhibits decreased solubility (viscosity) in aqueous salt solution. Thus, even in conventional techniques in which a sterilization step is employed, the number of the contaminative live bacteria in xanthan gum camnot be reduced to a satisfactory level.